Spec1 Phono Stage 2SA912?

[Overundr1]

The left channel in my spec1 phono stage , and only the phono stage not any of the other inputs, is prone to overload distortion and has got annoying enough now where I need to refresh the AWF-017 board While putting together a list of replacement transistors and caps I have run into a roadblock and could use a little help. The 2SA912 tranisistor subs to a 512-KSA910yshta but has been discontinued. Any ideas on what would be suitable? Once I get the unit out of my rig tomorrow I will do all the voltage checks but really suspect one of the gain transistors has gone belly up.
So far I have

2SA726 --- 512-KSA-992FBU
2SC1775 -- 512-KSC-1845EBU
2SC1885 -- 512-KSC-2310YBU
2SA912 -- ???

Thanks
-Lee

 

[avionic]

2SA912..... http://www.bdent.com/search/part.jsp?partnum=2SA912
2SA1124 will handle a little more power.

 

[EchoWars]

Replace Q7 (2SC1885) with a Fairchild KSC2682 or NEC 2SC2682, and Q9 with the Fairchild KSA1142 or NEC 2SA1142.

The NEC devices are tough to find. The Fairchild devices are at Mouser:
512-KSC2682YSTU
512-KSA1142YSTU

Edit: The A1124 & C2632 are good also, but are discontinued parts from Panasonic. Looks like B&D has them. $15 minimum order, I believe.

 

[markthefixer]

I'm not contradicting EchoWars here, those are good choices.

The difference between the discontinued 910 and the 992 is that the 910 had less gain and a larger case (higher voltage too), changing the power dissipation to .8w from .5w while still having a collector current of 50 mA.

I'm not sure how much power needs to be pumped out of the phono amp, so you MIGHT get away with 992's and 1845's everywhere in it..

BUT if the 992 & 1845 start to get warmer in the Q7 - Q10 positions, then go with the to-126 cased 1142's and 2682's.

2sa912 to-92 ecb 150v 100mA 0.75w 65-450hfe 200mhz

512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

512-KSA910YSHTA TO-92L ecb 150v .05a .8w 100mhz 40-240hfe discontinued :tears:
512-KSC2310YBU TO-92L ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea

512-KSA1142YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.28 ea
512-KSC2682YSTU to-126 ecb 180v .1a 1.2/8w 200mhz 100-320hfe $0.31 ea

 

[Overundr1]

Once again thanks for helping me out.
-Lee

 

[Overundr1]

2sa726 appears to be B-C-E in positions Q1-Q4
ksa992 appears to be E-C-B
thinking that I need to install the replacement transistors reversed
correct?
-Lee

 

[Overundr1]

Oh, I forgot to mention all the caps in the phono stage are now replaced and solder points touched up where needed. Enjoyed the bent over transistor mounting legs to circuit board immensely .So far so good.
-Lee

 

[Overundr1]

Spec1 back in service again. After replacing all the electrolytic caps and transistors in the phono stage I hooked the preamp up to my signal generator and scope. Traces are nice and sharp form 50 to 12000hz with no ripple. I find that the turntable now is balanced between the channels and the mid frequency harshness is gone. Whether it was old caps or a weak transistor I cannot determine but I am happy with the outcome and the repair was well worth the time. I was glad to get the transistor help along the way.

-Lee

 

[markthefixer]

Glad to hear that, just for grins, could you list the actual transistor substitutions in use, and if there were any heating issues on the output drivers.

thanks...

 

[Overundr1]

Mark--
2sa726 I used ksa992
2sc1775 I used ksc1845
2sa912 I used ksa1142
2sc1885 I used 2sc2682

Referring to the service manual I only ran the function generator at @+/-400mv level max while checking the output of the board. At that level the 1142 and 2682 did not seem to heat up. I cannot say what would happen if I pushed the input beyond 500mv or so, since I am still learning as I go along and did not want to create other issues to work on. Also a public shout out of thanks for answering my query on replacement vrs original pin configuration.
Been running the pre-amp for a couple of days now with no problems and much improved sound. Well worth the time to repair.
-Lee

 

[EJP286]

I used a BC560 with c/b legs reversed. The 2SA912 is ridiculously over-specified for this purpose at 150V.

 

[markthefixer]

I used a BC560 with c/b legs reversed. The 2SA912 is ridiculously over-specified for this purpose at 150V.

I respectfully disagree with your use of the BC560 in this spot.
.
The output push pull runs at +48v and -48v. 3.3mA idle current (0.4v / 120 ohms = 0.003333 A) and 0.3w worst case dissipation across the two transistors.
The BC560 has a Vceo of 45 volts.
IMO It's running right on the ragged edge.

REMEMBER, the last post with parts numbers is from 2009!!! the 2sc2682 which he meant to type ksc2682 has long been obsoleted.